Really fucking dumb. Uses browser geolocation to tell me which neighborhood in San Francisco I'm currently standing in. Grabbed the lat/longs of each neighborhood from the SFData [0]. It works by plotting a circle representing your location + uncertainty and determining which of SF's 117 neighborhoods intersect with it. If your circle intersects with more than one neighborhood, it's smart enough to categorize them in order of certainty. It even automatically updates as you move, it's pretty fun to sit in an Uber and watch the neighborhood change as you move across the city. Learned a lot about polygon intersection algorithms, they're pretty amazing!
Built it a number of years ago when I moved here. Since it runs entirely in the browser, it's hosted for free on github pages and I don't need to maintain a server.